The present invention relates to an in-vivo tomographic imaging system utilizing a nuclear magnetic resonance phenomenon, and more particularly to a phase distortion correcting method and apparatus suitable for attainment of phase distortion correction with high precision by use of only a tomographic or cross-sectional image.
In an inversion recovery (IR) method which is one of the magnetic resonance (MR) imaging methods, an image is acquired in which information concerning a parameter of an object called a longitudinal relaxation time is emphasized. A detection signal acquired by use of the IR method involves phase information. Therefore, this method has a problem in that the influence of phase distortion appears on a complex image reconstruction.
In a Dixon method of separating water and fat images from each other by use of chemical shift information, a difference in resonance frequency between water and fat is used to deliberately provide a phase difference between water and fat, thereby separating water and fat from each other. Since this method also uses phase information, it is affected by phase distortion.
Angiography for extracting the shape of blood vessels and blood flow measurement of measuring the flow rate of blood utilize the fact that the phase of a moving portion changes on a reconstructed image. Therefore, there is a problem that the presence of any phase distortion makes it difficult to correctly determine the shape of blood vessels or the flow rate of blood.
The conventional approaches of correcting phase distortion in a reconstructed complex image acquired in the above-mentioned IR method, Dixon method, blood flow measurement and angiography involve two methods, e.g. a method (1) in which phase distortion is calculated and corrected by preliminarily imaging a uniform object called a phantom in the same procedure as a procedure for acquiring a tomographic image, as disclosed by JP-A-61-194338, and a method (2) in which phase distortion is estimated and corrected through repetitive calculation (see "A Spatially Non-linear Phase Correction for MR Angiography", Sixth Annual Meeting and Exhibition, p. 29, 1987).
The method (1) has a problem in that the phase distortion generally changes depending on the time, the imaging means and the position of a tomographic image acquired, and therefore the imaging of the phantom has to be carried out frequently, thereby requiring a complicated operation and a long operation time. In the method (2), the phase distortion is corrected by use of only a tomographic image. However, since the phase distortion is estimated by use of the repetitive calculation, a large amount of calculation and hence a long time are required for estimation of the phase distortion. Therefore, the method (2) has a problem that the precision of correction is deteriorated since the optimum estimation value cannot always be determined in the case where the calculation is discontinued after a finite number of repetitions.